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Abundance of deuterium vs tritium

  1. Aug 15, 2005 #1
    In a nuclear fusion reactor for generating electricity, would a deuterium-deuterium reaction be inferior to a deuterium-tritium reaction? If the latter is a superior mode of fusion, then obviously that would be more preferable. But how abundant is tritium as an isotope, compared to deuterium, in natural water or ice? Far less, I would guess...

    AAI
     
  2. jcsd
  3. Aug 15, 2005 #2
  4. Aug 15, 2005 #3

    Morbius

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    AAI,

    It is easier to ignite a D-T reaction than the D-D reaction.

    Tritium has a natural abundance less than that of deuterium - but that is not a problem.
    One can manufacture the needed tritium by bombarding lithium with neutrons produced
    by the fusion reaction.

    Therefore, a fusion reactor can manufacture all the tritium it needs from a feedstock of
    lithium.

    Dr. Gregory Greenman
    Physicist
     
  5. Aug 15, 2005 #4

    Astronuc

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    Currently the USDOE produces its tritium in special absorber assemblies loaded in a commercial reactor operated by TVA - http://www.tva.gov/news/tritium.htm .

    As Morbius indicated, D+T reaction is easier to achieve than D+D, since it has a higher cross-section at a given temperature. D+D does produce, He3 + n or T+p, and the He3 and T can then be consumed in the D plasma (so-called catalyzed DD system).

    Also, T decays to He3 with a half-life of ~12.3 yrs.
     
  6. Aug 16, 2005 #5
    Hi to all,

    Yes, thanks for these replies. A couple of more things though...

    Is fusion likely to become THE next mode of choice for all worldwide electricity generation in the next 5 to 10 decades? How easy is deuterium-tritium fusion and, more importantly, how much more value for money would deuterium-tritiun fusion be over uranium fission reactors of present day?

    Finally, can it be said that any of these fusion reactors would be a better propulsive mass driver in a rocket ship used for space exploration? Would a deuterium-deuterium fusion reactor (the best ever achievable?) be better in a nuclear rocket motor than uranium-fission that can be achieved in the present times? If you wanted to move a massive object, like the International Space Station, from low Earth orbit of circa 400 km altitude to, say, all the way to the Moon... or even further out to say Mars... would a nuclear propulsion rocket utilising fusion be a better bet than one using uranium-fission?

    I'm talking about propelling HEAVY loads here...

    Thanks!

    AAI
     
  7. Aug 16, 2005 #6
    http://www.iter.org/ has a projected timeline for fusion for power generation. Given the size of the beast, I can't see the tokamak ever being suitable for a drive motor.

    http://www.fusion.org.uk/ has some useful information.
     
  8. Aug 16, 2005 #7
    The high improbability of controlled fusion ever becoming market-valuable

    Recent predictions of best-case scenarios put it at roughly 50% more-expensive than fission in the year 2050. The most appropriate application might be space propulsion since the fuel would be very light for any given amount of energy production.



    Because the health problems associated with spread of fission products have been recently solved, this is what I would specify for an Earth-to-orbit or planet-surface-to-planet-surface heavy lifter:
    en.wikipedia.org/wiki/Project_Orion
     
    Last edited: Aug 16, 2005
  9. Aug 16, 2005 #8
    Thanks a million Hitsquad. Yes, nuclear pulse propulsion using fission bombs is probably the best we'll have for as far as we could all see into the future.

    I am also told that only U-235 is fissionable, but its occurence as an isotope of uranium is less than 1% in nature (> 99% of it is U-238). So how easy and what process does one have to put U-238 through to get to U-235? Is it expensive?

    AAI
     
  10. Aug 16, 2005 #9

    Morbius

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    AAI,

    First - a correction in terminology. U-235 isn't fissionable - it is "fissile". That means it
    can be fissioned by neutrons of any energy - low energy or "thermal" neutrons included.

    http://www.nrc.gov/reading-rm/basic-ref/glossary/fissile-material.html

    U-238 is "fissionable". An isotope is "fissionable" if it will fission if the neutron has an
    energy above a threshold value. U-238 is fissionable - and the threshold is about 1 MeV.

    http://www.nrc.gov/reading-rm/basic-ref/glossary/fissionable-material.html

    Natural uranium is 99.3% U-238, and about 0.7% U-235. If you want to use the U-238
    as fuel - you don't convert it to U-235 - you convert it to Pu-239 which is also "fissile".
    All it takes to do that is to have the U-238 absorb a neutron - so you put it in a reactor.

    All reactors convert U-238 to Pu-239. In fact, in the 3 years that the average uranium
    fuel assemble spends in a commercial power reactor, about 40% of the energy you
    get out of that assembly comes from fissioning Pu-239 that was created in situ from
    U-238.

    Additionally, if you put your fuel in a "breeder reactor"; the breeder will convert more
    U-238 into Pu-239, than the amount of U-235 or Pu-239 that you fission. Therefore,
    you can use the 0.7% of natural uranium that is U-235 to "bootstrap" the process and
    burn 100% of the natural uranium. If we had breeder reactors, then we would be able
    to use the entire inventory of natural uranium as fuel - not just the 0.7 that is U-235.

    http://www.nrc.gov/reading-rm/basic-ref/glossary/breeder.html

    My favorite candidate would be Argonne National Laboratory's Integral Fast Reactor.

    Courtesy of PBS Frontline, an interview with ANL's Dr. Charles Till:

    http://www.pbs.org/wgbh/pages/frontline/shows/reaction/interviews/till.html

    Courtesy of the University of California - Berkeley:

    http://www.nuc.berkeley.edu/designs/ifr/

    The IFR is "inherently safe", proliferation resistant, and is a breeder.

    Dr. Gregory Greenman
    Physicist
     
    Last edited: Aug 16, 2005
  11. Aug 16, 2005 #10
    No, fusion bombs.
     
  12. Aug 16, 2005 #11

    Astronuc

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    ????

    As far as I know, distribution of fission products from atmospheric nuclear blasts would represent a considerable problem. Nuclear detonations for weapons testing were moved underground because of the fallout problem.

    Mankind does not need to be adding radionuclides to the environment. Infants and children are particularly susceptible to the effects of radiation, and it may take decades (or generations) for the effects to manifest themselves.

    I don't know of anyone seriously considering Orion systems in the Earth's atmosphere.
     
    Last edited: Aug 17, 2005
  13. Aug 17, 2005 #12
    PubMed vs the NRC

    The problem has been solved.
    ncbi.nlm.nih.gov/entrez/query.fcgi?CMD=search&DB=PubMed&term=radioprotective



    There seems to be something missing from your syllogism.



    Hi.
    https://www.physicsforums.com/member.php?userid=5571
     
  14. Aug 17, 2005 #13

    Astronuc

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    It is not clear to me that any problem is solved. Most of those articles deal with medical treatment after radiation damage (and the research is done on mice and rats!). I prefer avoiding radiation exposure in the first place - by keeping radioanuclides out of the environment.

    Yes the medical establishment can treat cancer - but that is hardly the solution. I'd rather people not get cancer in the first place. Children, particularly infants, are especially vulnerable to effects of radiation.

    Those of us who work in the nuclear industry work hard to prevent unnecessary releases of radioactivity into the environment. We have an ethical and moral obligation to protect the public. Every engineer assumes an ethical responsibility to protect society. Check the ANS code of ethics, and codes of ethics of any engineering/technical society.

    IMO, use of Orion in the earth's atmosphere is reckless, irresponsible, unethical and immoral. But then I like a clean environment. :smile:

    Fixed it.

    Hey. :smile:
     
  15. Aug 18, 2005 #14
    Thanks to all for these informed replies. I will be using this info in a future sequel of my *epic*, sci-fi novel about a life-like human colony starship voyage to Alpha Centauri, the next nearest system in space. Meanwhile, I just thought I should mention here, my first novel is actually releasing in the US right now. More details are on my publisher's website at:

    http://www.publishedauthors.net/aa_spaceagent/


    (PS: If you do get a chance to read the book, please be gentle on the reviews, as it's only my first time in sci-fi fiction...)

    AA
     
  16. Aug 18, 2005 #15
    Radioprotection vs repair

    I have not performed a comprehensive review of the articles, but I was not aware that any of them deal with anything other than prevention — rather than repair — of damage (though, the devil is in the details — there are many classes of chemical radioprotection from chelation to oxygen radical quenching to DNA strand repair — but even so, there is a discernable difference between protection by way of repairing DNA damage while it is occuring and repairing major organ damage long after the fact of radiation dosing). In those articles, statements like this one seem to me to be typical, "Radioprotective efficacy was evaluated by measuring the ability of the amifostine nanoparticles (equivalent to 500 mg/Kg) to inhibit whole-body gamma irradiation-induced injury in mice."

    Did you find one that deals with damage repair rather than damage prevention?
     
    Last edited: Aug 18, 2005
  17. Aug 20, 2005 #16

    Astronuc

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    It will take some time to browse those articles.

    Meanwhile there are several issues to consider regarding radiation exposure.

    1.) Infants and children with higher mitotic rates are much more susceptible to effects of radiation, and in the case of children or fetuses, damaged DNA early on can have a devastating effect, the earlier the worse.

    2.) The research is done on mice and rats, so how efficacious is it for infants and children, or adults.

    3.) The treatments may work for acute (one time) exposure, not chronic exposure. Isotopes, like Cs, Sr, or I taken into the body, may produce chronic exposure if not quickly removed.

    4.) The Pu pit of thermonuclear device is not completely consumed, so how many kilograms of Pu will be released to the enviroment in atomized form.

    Orion would contaminate the lauch site - for how long.

    The concept would have to be demonstrated in space first. The concept has only been demonstrated with chemical explosives on a small scale model.
     
  18. Aug 21, 2005 #17
    Is there such thing as a 'safe' radiation exposure limit for humans? How many rads can a human take (say an adult) before you can say for definite that he/she would suffer some kind of an ill effect? I think this was one of the debates the Apollo program managers had to contend with when planning Moon missions through the highly electrically-charged Van Allen radiation belts in the late 60s.

    Any thoughts?

    AA
    http://www.publishedauthors.net/aa_spaceagent/
     
  19. Aug 21, 2005 #18
    Probabilistic assessments are used for these types of things.
    http://en.wikipedia.org/wiki/LD50
     
  20. Aug 27, 2005 #19
    Depends on whether or not you believe the linear-no threshold model.

    LNT observes that the dose to consequence of high radiation doses (from atomic bomb survivors mostly) varies linearly. This is extrapolated down to a zero dose, meaning that any dose is theoretically harmful. It should be noted there is no direct physical evidence of the damage due to low dose mostly because it is very difficult to measure in a very large population. It is the most conservative of the theories out there. This is the current procedure for estimating effects of radiation by pretty much every organization due to its conservatism.

    Another theory says that the response is linear at high dose, but negligible at low dose, and hence a threshold. The basis being that the cellular repair mechanisms can adequately deal with the damage from low doses before any problems manifest themselves. The fact these mechanisms exist (which they do) make believe a truly linear response to be somewhat doubtful.

    The most liberal theory is hormesis. This theory states that small doses of radiation may actually be beneficial. The rationale for this is that low doses of ionizing radiation actually activate cellular repair mechanisms fixing problems that would have otherwise remained unfixed. There are plenty of experiments possibly demonstrating some positive effects, but the jury is still out on this theory, although it does seem to be becoming more likely.
     
  21. Aug 28, 2005 #20

    Morbius

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    BCRion,

    Check out:

    http://www.llnl.gov/str/JulAug03/Wyrobek.html

    which states,

    "Low-Dose Exposure Can Protect
    The team also discovered that the human lymphoblastoid cells exhibit what is
    called an adaptive response to ionizing radiation. An extremely low dose (also
    called a priming dose) appears to offer protection to the cell from a
    subsequent high dose (2 grays) of ionizing radiation. ....Regulatory agencies
    are convinced these effects do happen and that they may play a role in human
    health.”

    Dr. Gregory Greenman
    Physicist
     
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